The present invention relates to Fourier transform infrared spectrometer's (FTIR spectrometers) and, in particular, to a stage for use with such a spectrometer for making attenuated total reflectance (ATR) measurements.
Fourier transform infrared spectroscopy is a technique for studying the composition of matter by measuring the characteristic absorption of specific wavelengths of infrared radiation. The sample may be measured either with transmitted or reflected radiation.
In transmission spectroscopy, a beam of infrared radiation of known and time variant spectral composition is passed through a transmissive sample. The resulting transmission absorption spectrum is then compared to standard transmission absorption spectra to identify spectral absorption characteristics of the sample permitting identification of the sample's constituents.
With opaque samples, the technique of specular reflection spectroscopy may be used. In reflection spectroscopy, the beam of infrared radiation is directed against the surface of a sample at a predetermined angle of incidence. The spectrum of the energy reflected at an opposing reflection angle is then measured. As with transmission spectroscopy, the resulting reflection absorption spectrum may be compared to known reflection absorption spectra to reveal information about the composition of the sample or the coating of the surface of the sample. One type of specular reflection measurement is termed attenuated total reflectance (ATR) spectroscopy.
Making ATR measurements of a material involves placing the material against the sample surface of an infrared transmitting crystal, often referred to as internal reflection elements (IREs). Infrared light is introduced into the IRE crystal and made to internally reflect off the sample surface. During this reflection, the sample may absorb some of the energy of the infrared beam which interacts with the sample. A successful measurement requires that the sample be in intimate contact with the IRE since this interaction with the infrared beam only occurs over a few microns from its surface.
IREs that allow for a single internal reflection are called single-bounce IREs. Single-bounce IREs may come in a variety of shapes including trapezoidal shape prisms, cylindrical rods or hemispherical domes. They offer the advantages of requiring only a small sample and of needing only low clamping forces to hold the sample against the sample surface. The drawback to single-bounce IREs is that there is only one interaction between the sample and crystal and the resulting spectrum may be weak.
In order to increase the light energy coupled to the sample, multi-bounce IREs may be used. In multi-bounce IREs, multiple reflections occur against the side of the crystal contacting the sample to increase the amount of attenuation and improve the obtained spectra. Multi-bounce crystals require that the sample be held in close proximity to the crystal over a relatively broad area which increases the total force that must be applied to the sample.
Single and multi-bounce crystals are typically limited to having a single transmissive element so as to reduce Fresnel reflection losses occurring when light passes into or out of a transmissive element. One problem with both types of crystals is that not all the light passing through the IRE strikes the sample surface. The light that does not strike the sample surface is commonly termed stray light and may distort the spectral features that are being investigated.
Additionally, solid samples must be pressed against the IRE, especially in multi-bounce systems. If a solid sample is rough or abrasive it may damage the sample surface of the IRE. Minute nicks and scratches in the sample surface of the IRE can cause optical anomalies which degrade the performance of the spectroscope.
Hard materials with the proper spectral characteristics, such as diamonds, can provide a suitable scratch-resistant surface on which to place a solid sample. The size and cost of diamonds, however, effectively prohibits diamonds from being made into focusing lenses. And, cost notwithstanding, its hardness makes it difficult to machine the convex and/or concave surfaces needed for a lens.